Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
In some electrical heater applications, it may be desirable to vary the watt density of the heater in certain areas in order to tailor the amount of heat delivered to the specific part or device being heated or to account for inherent variations in heat distribution along the heater trace or element. Known electrical heaters typically vary the spacing of the resistive circuit pattern such that where the spacing is smaller and the trace of the resistive circuit pattern is closer, the watt density is higher, for a series circuit configuration. Conversely, the larger the spacing between the traces of the resistive circuit pattern, the lower the watt density in those regions. In other known electrical heaters, the width of the trace of the resistive circuit pattern is varied along its length in order to vary the watt density, wherein the wider the trace the lower the watt density and the narrower the trace the higher the watt density, again, for a series circuit configuration.